1. Field of the Invention
The present invention relates to an optical disc apparatus in which a laser beam from an optical pickup is shone onto a recorded surface of an optical disc and information is read from the optical disc, a method for adjusting a servo of this optical disc apparatus, a computer-readable recording medium recording a servo adjustment program, and to a servo adjustment program.
2. Related Art
Optical disc apparatuses which accommodate various types of optical discs, such as a CD (Compact Disc), DVD (Digital Versatile Disc), CD-ROM (CD-Read Only Memory), DVD-ROM (DVD-Read Only Memory), and CD-R/RW (CD-Recordable/CD-ReWritable) in general have an optical pickup, which shines a laser beam onto a recorded surface and which performs reading from and writing to the disc of a signal, and an optical pickup moving mechanism, which causes the optical pickup to move to or to near to a desired track on the optical disc.
Of such optical disc apparatuses, there are types that have an automatic adjustment mechanism, which adjusts the relative distance (focus) or the relative angle (skew angle) between the optical disc and the optical pickup.
There are a variety of methods of achieving the above-noted automatic adjustment, and example of which being one whereby constants establishing such servo characteristics as skew angle or focus bias are changed by a prescribed step as variables and jitter values (jitter values of the RF signal read from the optical disc) are measured in correspondence to the changing variable values at each prescribed step are measured, the values of constants determining the servo characteristics being optimized based on the measured jitter values.
More specifically, in the above-noted method, for example as illustrated in FIG. 1, a jitter value that is larger than a minimum value Jal obtained by past measurements by a reference amount α is set as the jitter threshold value Jαth, and each jitter value (shown by the curve L in the drawing) are measured as the constant (SV) which establishes a servo characteristic is sequentially varied from a small value to a large value by a prescribed step, and of these jitter values a first variable value SVa corresponding to a first jitter value at point A in the drawing, which exceeds the jitter threshold value Jαth, and a second variable value SVb corresponding to a second jitter value at point B in the drawing are determined, and further a third variable value SVc, which is for example an intermediate point between the first variable value SVa and the second variable value SVb, is set so as to optimize a constant value establishing the above-noted servo characteristic. The above-noted constant for establishing a servo characteristic corresponds to a relative distance or relative angle between the recording surface of the optical disc and an optical pickup. The above-noted reference amount α is, for example, a fixed value established based on an upper limit value (for example the jitter threshold value Jαth for which a data error can be corrected when reading from the optical disc.
The above-noted jitter threshold value Jαth, as noted above, is set as a value that is an amount α greater than the minimum value of jitter Jal measured in the past, and if the fixed reference amount α is priorly set to a small amount so that the jitter threshold value Jαth is a small value, it is possible to shorten the amount of time to measure the above-noted optimum value from the first variable value SVa and the second variable value SVb. That is, in the case in which the first variable value SVa and the second variable value SVb are determined from the jitter values measured while the variable SV is sequentially changed from a small value to a large value, if the above-noted jitter threshold value Jαth is made a small value, the second variable value SVb is obtained particularly quickly, making it possible to quickly determine the above-noted third variable value SVc (optimum value).
There, however, are variations in the reflection characteristics of the recording surface of the optical disc, and variations in the characteristics of the various constituent elements of an optical playback apparatus. For this reason, because of the existence of these variations, even for the same value of constant that establishes the servo characteristic, the jitter value occurring in the RF signal is often different, there being cases in which the value is better and those in which the value is worse. In particular in the case in which the jitter value is worse, as shown by the curve Ld in FIG. 2, not only is there a rise in the jitter at the lowest part, but also there is an increase in the amount of change in the jitter value in response to the above-noted change in variable values.
Therefore, in the case in which, in order to shorten the time for setting the optimum value, the above-noted fixed reference amount is made a small value αd from the start, such as shown in FIG. 2, so that the jitter threshold value is a small value of Jαdth, if the jitter value worsens, such as shown by the curve Ld in the drawing, there is a risk that it will not be possible to achieve a first variable value SVad and a second variable value SVbd sufficient to set the above-noted optimum value. In particular in the case in which, for example, there is external disturbance, such as flaws in the optical disc or vibration or the like occurring during the jitter measurement, a suddenly occurring variation D will occur in the jitter value shown by the curve Ld, and if the jitter value of this variation D exceeds the jitter threshold value Jαdth, there will be an erroneous detection of the variable value SVd responsive to the jitter value of the variation D as the second variable value. As a result, the intermediate point variable value SVe between the erroneously detected variable value SVd (erroneous second variable value) and the first variable value is erroneously set as the optimum value of the constant that establishing the servo characteristic (that is, the third variable value).
If, for example, the reference amount α is made a large amount αg at the start, so that the jitter threshold value is a large value Jαgth, it is possible to obtain a first variable value and a second variable value sufficient for setting the above-noted optimum value. Thus, if the jitter threshold value Jαgth is used, even if a jitter variation D occurs suddenly, such as shown by the curve Ld in FIG. 2, because there is no erroneous detection of the variable value corresponding to the jitter value of the variation D as the second variable value, it is possible to set the intermediate point variable value (third variable value) between a non-erroneous second variable value and a first variable value as an optimum value of a constant for establishing the servo characteristic.
However, if the fixed reference amount α is priorly set to a large amount αg so that the jitter threshold amount is a large amount Jαgth, in the case in which the jitter is good, such as shown by the curve Lg in the drawing, for example, excessive time is required for the measurement of the above-noted optimum value. That is, when the jitter value improves, the curve Lg in FIG. 2 not only exhibits a reduction in the jitter at the lowest part, but also a reduction in the amount of change in the jitter with respect to the amount of change in the variable values. Because of this, there is a broadening of the span between the minimum jitter value Ag that exceeds the jitter threshold value and the maximum jitter value Bg, the time required to obtain these values of Ag and Bg becoming long, thereby resulting in an extremely long time required to obtain the third variable value SVf, which is the intermediate point between the second variable value SVbg and the first variable value SVag.